The present invention relates to plasma display panels (PDPs). More particularly, the invention relates to PDPs having at least one constituent layer whose structure is porous.
Plasma display panels are flat display screens in which the displayed image consists of a set of luminous discharge points. The luminous discharges are produced in a gas contained between two insulating plates. Each discharge point is generated by a discharge cell defined by a point of intersection in arrays of electrodes carried by at least one of the plates.
Thus, a PDP comprises a two-dimensional matrix of cells, which is organized in rows and columns copied from the geometry of the electrode arrays. Relief elements, called barriers, may be placed so as to separate the cell rows or the cell columns. In some panels, the barriers may also separate both the cell rows and the cell columns, thus forming a grid pattern of the latter.
The barriers have several functions. By partitioning the space of each cell, at least in the direction of the rows or the columns, the barriers prevent a discharge in one cell causing undesirable discharges in adjacent cells by an ionization effect. They thus prevent crosstalk phenomena.
Moreover, the barriers constitute optical screens between the adjacent cells, making it possible for the radiation emitted by each cell to be well confined in the space. This function is particularly important in colour PDPs in which the adjacent cells constitute respective elementary dots of different colours, for example in order to form triads. In this case, the barriers ensure good colour saturation.
Finally, the barriers often serve as a spacer between the two plates of the panel. What is exploited in this case is the fact that the barriers can have a height which corresponds to the required separation between the two plates and that they are uniformly distributed over the useful area outside the discharge points. In this case, the plate not provided with barriers rests on the tops of the barriers that are present on the other plate. There are also panels in which barriers are present on each of the plates, the plates being joined together with the barriers top against top.
FIGS. 1 and 2 illustrate an AC colour plasma display panel having a so-called coplanar structure, according to a known architecture.
The PDP comprises a first glass plate 2 and a second glass plate 4 a few millimetres in thickness, these being placed face to face with a separation of the order of 100 microns between the internal faces when they are joined together (FIG. 2).
The first plate 2 has, on its internal face, an array of parallel electrodes grouped in closely spaced pairs of electrodes Y1a-Y1b, Y2a-Y2b, . . . , Y5a-Y5b, etc. Each pair of electrodes constitutes a display row of the panel. The electrodes are embedded in a thick layer of dielectric material 6, for example glass, which cover the entire useful area of the plate 2. This layer 6 is itself covered with a thin layer 8 (less than 1 micron in thickness) of another dielectric material, in this case magnesium oxide (MgO), the surface of which is exposed to the discharge gas.
In the example, the internal surface of the first plate 2 may, for example, be provided with a contrast-improving matrix 10. The said matrix consists of a mosaic of elementary colour filters surrounded by generally black rings.
The second plate 4 has, on its internal face, an array of uniformly spaced parallel electrodes X1, X2, . . . , X6, etc., perpendicular to the row electrodes Y1a-Y1b, Y2a-Y2b, . . . , Y5a-Y5b, etc., which constitutes the address electrodes of the plasma display panel. As in the case of the first plate 2, these electrodes X1, X2, . . . , X6, etc. are embedded in a thick dielectric layer 12 which is itself covered with a thin layer of magnesium oxide 14.
A discharge cell of the PDP is thus formed by the intersection of an address electrode X1, X2, . . . , X6, etc. with a pair of electrodes Y1a-Y1b, Y2a-Y2b, . . . , Y5a-Y5b, etc. of a display row.
In operation, an AC voltage, called a sustain voltage, is applied between the electrodes forming the pair of electrodes of each display row. The discharges are produced on the surface between these electrodes according to a voltage signal applied to the address electrode, using well-known multiplexing techniques.
It is especially possible to modify the luminous discharge state of each cell using row-by-row scanning in order to produce a display in video mode.
Straight barriers 16 are placed on the thin layer 14 of the second plate 4 at each place between adjacent address electrodes X1, X2, . . . , X6, etc. and parallel with the latter. The barriers 16 have walls perpendicular to the surface of the plate 4 and a flat top serving as a bearing surface for the internal face of the first plate 2. In some constructions, the barriers may be of trapezoidal cross section so as to improve the luminous intensity. They thus partition the discharge cells in the direction perpendicular to the address electrodes X1, X2, . . . , X5, etc. and serve at the same time as a carrier structure for the spacing of the two plates 2, 4.
Typically, the barriers 16 have a height of the order of 100 microns and a pitch of 220 microns for a 50 micron width.
Phosphors 18R, 18G, 18B are placed in stripes on the exposed surface of the second plate 4. A phosphor stripe covers one surface portion of the thin magnesium oxide layer 14 bordered between two adjacent barriers 16. It also covers the perpendicular walls of the two barriers 16 which are turned towards this surface portion. Each phosphor stripe 18R, 18G, 18B has its own elementary emission colour among red, green and blue in response to a luminous discharge (generally in the ultraviolet) received from a cell. Together, the phosphors constitute a repeat pattern of three successive stripes each having a different emission colour so that a succession of elementary colour triads are created in the direction of the address electrodes, X1, X2, . . . , X5, etc.
The two plates 2 and 4 are sealed together and the space that they contain is filled with the discharge gas at a low pressure, after vacuum pumping through a stem.
It should be noted that the presence of the layers of dielectric material 6, 8 and 12, 14 on top of the electrodes Y1a-Y1b, Y2a-Y2b, . . . , Y5a-Y5b and X1, X2, . . . , X5, etc. is characteristic of AC PDPs. The dielectric material forms with the electrodes a capacitor across which is applied, in the gas, the voltages necessary to generate and sustain the luminous discharges.
An advantageous feature of AC PDPs is that the AC sustain voltage automatically fixes the state of a luminous discharge point from the last command received, namely either the luminous discharge is maintained or it remains absent, depending on the command previously transmitted. This thus results in an inherent image memory effect, hence the possibility of addressing the points only when their luminous state has to change.
FIG. 3 shows another example of an AC PDP, this time with a matrix structure. This type of PDP differs from coplanar panels essentially by the fact that the discharges are produced between the respective surfaces of the two facing plates 2 and 4.
The components that are analogous between this panel and the one described previously bear the same references.
As in the previous case, the PDP comprises a first plate 2 and a second plate 4, each provided with an array of mutually parallel electrodes Y1, Y2, Y3, . . . , Y7, etc. and X1, X2, X3, . . . , X7, etc. which are embedded in a thick layer of dielectric 6 and 12, this layer itself being covered with a thin layer of magnesium oxide 8 and 14. For both plates, the pitch between the electrodes is in the order of 0.5 mm.
The array carried by the first plate 2 constitutes the row of electrodes Y1, Y2, Y3, . . . , Y7, etc., each display row being associated with a single electrode.
The array carried by the second plate 4 constitutes the column electrodes X1, X2, X3, . . . , X7, etc., these being placed so as to be perpendicular to the row electrodes.
The second plate 4 also includes a system of barriers 16 in the form of a thick layer (of the order of 100 microns in thickness) in which wells 20 are formed. The wells 20 pass right through the thickness of the layer which constitutes the system of barriers 16 and thus expose the thin MgO layer 14.
When the two plates are joined together, the first plate 2 bears on the layer of barriers 16 via balls 17.
The wells 20 are distributed in a staggered pattern and are centred on crossover points between the row electrodes Y1, Y2, Y3, . . . , Y7, etc. and the column electrodes X1, X2, X3, . . . , X7, etc. In the case illustrated, two adjacent row electrodes Y2i and Y2i+1 form a pair and receive the same electrical signal. The wells 20 have a circular cross section of average diameter of the order of 0.5 mm. Each well 20 forms a discharge cell with the crossover of the row electrode and the column electrode with which it is associated. The staggered distribution of the wells 20 means that, along each row electrode Y1, Y2, Y3, . . . , Y7, etc. there is, in succession, one discharge cell per two points of crossover with the column electrodes X1, X2, X3, . . . , X7, etc. Likewise, along each column electrode there is, in succession, one discharge cell per two points of crossover with the row electrodes. Thus, 50% of the electrode crossover points on the plate assembly constitute discharge points. According to another construction, it is known to use zigzag electrodes, and in this case half the number of electrodes are used.
Each luminous discharge therefore is produced within a well 20 between the respective exposed MgO layers 8 and 14 of the two plates 2 and 4. The discharge cells are thus perfectly partitioned both in the row direction and in the column direction.
In order to obtain a colour display, phosphors are introduced into the wells 20, each well having a phosphor of primary emission colour different from that of the adjacent wells so as to produce elementary triads in a repeat pattern. The phosphors occupy an annular volume in their wells 20, the central space being left clear in order to create the luminous discharges.
It should be noted that the system of barriers 16 having staggered wells 20 occupies a large part (40 to 60%) of the total area of the second plate 4. It thus allows a strong and stable carrier structure for receiving the spacer balls supporting the first plate 2 to be readily produced.
U.S. Pat. NO. 4,037,130 teaches those skilled in the art that in order to obtain optimized barriers it is preferable for these to be porous. The porosity of the barriers is combined with a gettering effect of the material of which the barriers are composed, in order to remove any parasitic gases that may remain in the panel after pumping.
The gettering effect is a surface absorption property specific to certain materials which can trap certain molecules on their surface. The combination of the gettering effect with the porosity of the material used means that the risk of obtaining a defect due to the outgassing of the materials is almost zero. It is also possible in a plasma display panel to deposit a layer of a gettering material other than the barrier layer.
Whatever the structure used for the barriers 16 or a possible gettering layer, the layer must be made of a hardened material. This is also necessitated when the layer is a layer of barriers intended to be bearing barriers.
This is because bearing barriers have to be able to withstand the considerable force exerted by atmospheric pressure on the plates. During the operation of vacuum pumping the space between the plates 2 and 4 before introducing the low-pressure discharge gas, the force exerted per unit area of barrier may be as much as 106 pascals (approximately 10 kg/cm2) depending on the ratio of the bearing area of the barriers to the total area of the panel.
In the prior art, the barriers 16, like those described with reference to FIGS. 1 to 3 for example, includes an hardening agent, generally a glassy phase, which is sufficiently crush-resistant to maintain a constant space between the two plates.
These barriers are produced, for example, by the screen printing (in 10 to 20 successive layers) of a paste containing a glass frit or by the sand blasting of a layer containing a glass frit.
After producing the barrier geometry, these layers are fired at temperatures of between 450 and 600xc2x0 C. so as to solidify the hardening agent and make the layer comprising it mechanically strong.
The invention aims to improve the effects of the barrier layers as well as any layers of gettering material by increasing their porosity so as to eliminate any undesirable outgassing problem after vacuum pumping.
The subject of the invention is a plasma display panel consisting of two facing plates enclosing a discharge space comprising an array of discharge cells, the said panel including a layer of a material which contains less than 10% of a hardening agent. The layer in question is either a barrier layer or, more generally, a layer of a gettering material.
The use of less than 10% hardening agent results in the layer in question being softened. Contrary to what those skilled in the art might think, such softening makes is possible, on the one hand, to ensure the cohesion of the layer and, on the other hand, to guarantee the spacing in the case of bearing barriers. A hardener content of 5% to 10% makes it possible to achieve a compressive strength of the order of 106 pascals (10 kg/cm2). This case corresponds to barriers which cover only 15 to 25% of the area of the panel, this generally being the case in structures such as those described with reference to FIGS. 1 and 2.
According to one particular embodiment, the panel includes a layer of barriers defining staggered wells, the barriers extending from one plate to the other. By reducing the content of hardening agent, the barriers may be sufficiently porous to allow pumping through the barrier layer. The use of a structure consisting of bearing barriers with staggered wells furthermore makes is possible to have a large bearing area, allowing the content of hardening agent to be reduced. It is possible to use a layer containing less than 4% of hardening agent. According to one particular embodiment, no hardening agent is used.
The structure comprising staggered wells and bearing barriers no longer uses balls. It is possible to make use of spacers produced above the barriers, or on the opposite plate, made of the same type of material. Such a technique makes is possible, in particular, to improve the production efficiency by eliminating defects due to the balls rolling while the plates are being joined together.
Moreover, since the barriers contain little hardening agent they have a relatively low density, thereby conferring on them a capacity of undergoing localized compaction when stressed. This characteristic is advantageous when the barriers are bearing barriers. In this case, the plate bearing on the tops of the barriers will level out all the overthicknesses formed during the vacuum treatment (vacuum-pumping cycle), by localized densification of the material.
It is therefore possible to obtain well-controlled spacing of the plates without having to make use of specific techniques aimed at producing great uniformity in barrier height.
In contrast, the relatively rigid barriers used in the prior art must either be dressed or be produced in a process which gives very good height uniformity. This is because any height non-uniformity causes a variation in the spacing between the plates if the barrier is quite solid or causes the barriers to shatter, which may damage the phosphor coatings.
The barriers according to the present invention are preferably composed of a material containing a mineral filler in the form of powder.
In order to ensure maximum porosity while still having good adhesion, the mean elementary diameter of the powder particles preferably lies within the 1 to 20 micron range, and even more preferably the 5 to 8 micron range. A preferred threshold corresponds to a filer consisting of a powder 90% of the mass of which has a particle size such that the particle diameter is greater than 2 microns.
It has been found that a narrow particle size distribution with a mean diameter approximately between 5 and 8 microns is very suitable and gives the deposited layer good cohesion. Barriers resulting from this choice of particle size may withstand a pressure of 5xc3x97105 pascals (approximately 5 kg/cm2) without the addition of a further component and exhibit maximum porosity.
By way of indication, such a compressive strength is sufficient to allow the production of bearing barriers if the latter cover one quarter or more of the area of the panel. This is especially the case for a system of barriers with staggered wells.
Preferably, the layer includes a filler composed of at least one oxide from among: aluminates, alumina, yttrium oxide, yttrium borate, clays, calcium oxide, magnesium oxide, titanium oxide, zirconium oxide or silica. The choice of one or more of these oxides will depend on the gettering effects specific to the materials.
The hardening agent may be a glass, such as a lead or bismuth borosilicate, having a softening temperature below the temperature of the heat treatment or treatments undergone subsequently in the process (lying between 380 and 500xc2x0 C.).
The hardening agent may also be a silicate, such as sodium silicate, potassium silicate or lithium silicate, etc., or a phosphate, or a carbonate, or a glass based on an oxide of tellurium of silver and vanadium, or else potassium dichromate.
During the heat treatment to burn off the organic binders needed for using these materials, typically comprising a treatment at 380-500xc2x0 C. for 0.5 to 1 hour, the hardening agent softens or melts and binds the filler particles together, forming bridgings, without creating closed porosity. In the case of silicates, the filler particles are bonded together.
The invention allows all the techniques conventionally used for producing the barriers, such as screen printing, sand blasting, photolithography, etc.